It is common practice in the wire and cable industry to manufacture wire strands and to amass a generally circular central assembly or bundle of elements for cable and to extrude an insulating layer over the assembly. Sometimes, the central bundle of elements is spirally wrapped with a tape to provide a continuous conductor shield bonded to the insulation layer. The tape itself may be overlaid with a semiconducting material or only the overlaid semiconducting material may be used to dissipate the electric stresses emanating from the conductors and prevent concentration of stress in the insulation layer formed about the conductor. In prior art cables, it was deemed that such semiconducting layers should preferably have a regular outer form, and specifically a circular cross-section. The circular cross-section was deemed necessary to minimize the stress concentration at the surface of the semiconductor and prevent such stress from impairing the properties of the insulating layer formed thereabout. The Association of Edison Illuminating Companies has established the standards known as AEIC5 and AEIC6, concerned with the acceptable height of spikes permitted in cables having different diameters and different insulation thicknesses. These standards are similar to the ICEA standard S-66-524 of the Insulated Cable Engineering Association.
It has been pointed out that there are problems associated with extruded semiconducting layers which concern principally the spikes or points of such semiconducting layer which extends into the insulating layer.
In the formation of medium voltage cable, particularly in a range of 5 kilovolts to 35 kilovolts, it has been observed that a nylon base semiconductive tape of butyl composition may give rise to a layer of tape which is deficient or even defective, i.e. tape wrinkles or is not bonded to insulation layer. Also, the extrusion of a semiconducting layer onto an inner conductor, particularly a stranded conductor, involves the use of two extrusion machines in sequence and, accordingly, is a more expensive operation than the use of semiconducting layer about the conductor of a cable. Use of an extrusion machine to produce conductor shield requires a high degree of skill and may result in excentric extrusions. The problem of obtaining concentricity can interfere with the production of suitable cable.